Robust ranging method

ABSTRACT

In a communication system in which data is transferred by packets, a ranging method in which a receiver, in a given ranging window, periodically compares received data with expected data to find a match. The periodic comparison includes searching for known preamble and/or delimiter sequences of ranging packets and involves timeouts for each search period. In case a match between the known sequences and received sequences is not found and the respective timeout is exceeded, the search and comparison process is restarted and continues until a global timeout is exceeded.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a Continuation of pending U.S. patent application Ser. No.11/856,735, filed Sep. 18, 2007, which claims priority from U.S.Provisional Application No. 60/825,931, filed Sep. 18, 2006.

FIELD OF THE INVENTION

The present invention refers in general to Time Division Multiple Access(TDMA) communications and in particular to Ranging used in TDMAcommunications

ACRONYMS

AFE—Analog Front End. The AFE consists of a transceiver, atrans-impedance amplifier, a limiter amplifier and Clock Data Recovery(CDR) component.

OLT—Optical Line Termination ONU—Optical Network Unit MAC—Medium AccessControl TDMA—Time Division Multiple Access CDR—Clock and Data RecoveryTO—Time Out AGC—Automatic Gain Control BACKGROUND OF THE INVENTION

Modern DMA communication systems are known in a variety ofconfigurations. FIG. 1A shows a typical point to multi-pointcommunication system in which a receiver 102 (see also FIG. 3)communicates bi-directionally with a plurality of users 104-1 to 104-N,FIG. 1B shows data packets (in order of users 1, 2, 3 . . . N) arrivingin order at the receiver. FIG. 2 shows schematically a particular,passive optical network (PON) communication system, in which thereceiver is represented by an OLT 202 and the users are represented byONUs 204-1 to 204-N. The PON may be of any known type, for example aGigabit-capable Passive Optical Network (UPON) or Ethernet-capablePassive Optical Network (EPON)

A typical receiver 300 is shown in FIG. 3. The receiver can bothtransmit and receive. It includes an AFE (receiving analog front end)component 302, a CDR (receiving clock and data recovery) component 304 alogic unit (MAC) 306 and a transmitter TX 308, interconnected as shown.AFE 302 amplifies the low power data in the receive side. A firstcontrol signal (Reset AFE) 310 is supplied by the logic unit to assistthe AFE in its functions, This Reset AFE signal assists the AFE to tuneto the incoming data. The CDR locks to the input data. It acquires thefrequency and the phase of an input data 312 signal and outputs adigital Received Data signal 314, synchronized with a Received Clocksignal 315, to the logic unit. A second control signal (Reset CDR) 316is supplied by the logic unit to assist the CDR in its functions. ThisReset CDR signal assists the CDR to fast lock (frequency and phase) tothe input data. The logic unit performs all the processing and controlsthe Ranging processes (explained below).

Ranging is performed in multi-user networks, in which the packet arrivaltime should be accurate and much smaller than the propagation delay. Ingeneral, in such networks, the Ranging targets include detection of anew user, synchronization of the receiver with the incoming bit streamof the new user; and estimation of the propagation delay (or round tripdelay) of the new user. After the Ranging process is successfullyfinished, the arrival time of the new user's packet is known to thereceiving system, and receiving from the new user can be accuratelyinitiated by the receiver.

FIG. 4 shows a typical situation when a ranging window is allocated fora new user to join the network. A ranging window is a period of time inwhich all the new users can try and join to the network (in order not tocollide with other users during Steady State operation, defined as theoperation mode after successful completion of Ranging. The packets ofdifferent users may have different propagation delays (or round tripdelays). In order to prevent contentions, the receiver must synchronizethe different users. Before Ranging, the start of packet arrival timefrom the new user is unknown to the receiver. Although it is performedonly when a new user joins the network, Ranging is a difficult task andneeds to be carefully addressed. Various Ranging methods (or“processes”) are exemplarily described in U.S. Pat. Nos. 6,980,561,6,948,184, 6,768,730, 6,215,792, 5,802,061, 7,251,240, 7,016,355,6,853,624, 5,850,525, 5,379,299, 5,043,982 and 4,845,735, allincorporated herein by reference in their entirety.

FIG. 5 shows a common packet structure, as known in the art. A packetincludes, in order, a Guard Time section 502, a Preamble section 504, aDelimiter section 506 and a Payload section 508. The Guard Time,Preamble and Delimiter sections are sometimes named “Header”. The GuardTime is a time period in which no energy is transmitted. The Preamble(or synchronization sequence) is a sequence zeros and ones. The sequenceshould consist of a lot of transitions to help the CDR to lock. TheDelimiter is a sequence of bits which must be different from thePreamble. Its main use is to assist in detecting the beginning of thePayload. The structure of the Header is defined to enable the clockrecovery to lock on the right timing.

FIG. 6 shows a straightforward, prior art Ranging method, typical ofthose indicated in the abovementioned references. At the beginning of aranging window, the receiver activates the AFE and starts looking for aPreamble and Delimiter of a Ranging Packet until it finds both. In theRanging process, since the arrival time is not known, the Header islarger than in Steady State, in which the Header should be as small aspossible. This is possible since the arrival time is known to thereceiver. The straightforward. Ranging process has no periodicity init—it occurs once. The process is finalized (ends) when the Delimiter isfound and the receiver starts receiving the data correctly. Thisstraightforward method assumes that the AFE does not require specialcontrol signals synchronized with the incoming data, which is usuallythe case.

To summarize, this kind of Ranging is limited to only few cases and itcan be used only:

-   1. when the AFE and the CDR can detect data without external control    signals (usually with special timing requirements (the APE and the    Burst Mode CDR sometimes require that the control signals should be    activated during the preamble reception)-   2. when the AFE blocks the data when the signal is low (squelch).    Otherwise, false alarm reception can mislead the Logic Unit. This    requirement requires special hardware in the AFE, the CDR or both    components.

The main drawback of this method appears when there is a chance of falsealarms or when one of the above conditions is not fulfilled. If there isa false alarm, there is no second chance, the burst is not received andthe Ranging needs to start again. Moreover, some types of receivers mustbe activated during the reception of data, so the above Ranging methodcan not work at all.

Usually, to enable the Ranging process, special hardware building blockssuch as a Fast Power Detection flag, a Fast AGC (Automatic GainControl), etc., are built in the front end of the system. These buildingblocks are expensive and suffer from limited performance in terms ofprobability of detection versus false alarms. They also requireexpensive calibration. Moreover, most communication receivers, forexample Burst Mode optical receivers (optical receivers that areintended for burst operation) or Burst Mode CDRs require a reset signalto start a proper receiving operation. This reset is possible only afterthe Ranging process since the packet arrival time must be known to thereceiver. Furthermore, in a noisy environment, the false alarm rateincreases, which sometimes makes Ranging (using such hardware block)impossible.

Accordingly, there is a need for, and it would be advantageous to have arobust Ranging method, which requires no additional hardware for itsimplementation (except for the ability to receive data), and which canalways guarantee successful Ranging.

SUMMARY OF THE INVENTION

The present invention discloses a “periodic” Ranging method that enablesuse of simple and inexpensive AFE and CDR components. Its specialinventive aspect of “periodicity” is explained in detail below. Inparticular, a receiver periodically supplies control signals to both theAFE and the CDR, although the arrival time of a new packet is not knownto the receiver. The logic unit (MAC) receives a sequence of bits fromthe CDR (which can be meaningless if the control signals are not in theright place) and compares it with an expected (i.e. known) Preamble. Incase such a Preamble is not detected within the first timeout, thesearch process repeats (APE and CDR activation, looking for thePreamble). If the Preamble is detected, the receiver searches for adelimiter. If the Delimiter is detected within the second timeout, theRanging process is done. If not, the receiver starts looking for thePreamble again, etc. This periodic or “cyclic” process continues untilboth the Preamble and the Delimiter are detected or until a globaltimeout (preferably different than the first two) is exceeded. After theDelimiter is detected, the arrival time of the packet is known andpropagation delay estimation can be performed.

According to the present invention there is provided a ranging methodcomprising the steps of: after start of a ranging window, periodicallycomparing received data with expected data to find a match therebetween;in case of a match failure, restarting the comparison within the sameranging window until a match is found; and if a match is found, movingto a steady state operation regime.

In some embodiments, the step of periodically comparing received datawith expected data includes searching an incoming data stream for aknown packet preamble until the preamble is found or until a firsttimeout is exceeded, whereby the exceeding of the first timeoutrepresents a first match failure.

In some embodiments, the step of periodically comparing received datawith expected data includes searching an incoming data stream for aknown packet delimiter until the delimiter is found or until a secondtimeout is exceeded, whereby the exceeding of the second timeoutrepresents a second match failure.

In some embodiments, if the preamble is found, the step of periodicallycomparing received data with expected data further includes searchingthe incoming data stream for a known packet delimiter until thedelimiter is found or until a second timeout is exceeded, whereby theexceeding of the second timeout represents a second match failure.

In some embodiments, the step of periodically comparing ends in a matchfailure and the step of restarting includes restarting the comparinguntil a global timeout is exceeded.

In some embodiments, the step of restarting includes activating ananalog front end unit of the receiver.

In some embodiments, the global timeout equals a ranging window.

According to the present invention there is provided a ranging methodcomprising the steps of: after start of a ranging window, searching fora ranging packet section until the ranging packet section is found oruntil a respective timeout is exceeded; if the packet section is notfound or if the respective timeout is exceeded, checking if a globaltimeout TO3 is exceeded and if not; repeating the steps of searching andchecking within the same ranging window until global timeout TO3 isexceeded.

In some embodiments, the method further comprises the step of: if thepacket section is found in the step of searching, moving to a steadystate operation regime.

According to the present invention there is provided a ranging methodcomprising the steps of: starting a ranging window, and, within the sameranging window, performing an iterative search and comparison process onincoming packet data, the iterative process involving a ranging packet,until at least one ranging packet section is matched with at least onereceived packet data section or until a global timeout TO3 is exceeded.

In some embodiments, the step of performing an iterative search andcomparison process includes using a respective timeout at anintermediate stage of the ranging window if a match is not found betweenthe at least one ranging packet section and at least one received packetdata section in that intermediate stage.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is herein described, by way of example only, withreference to the accompanying drawings, wherein:

FIG. 1A shows schematically a typical, known, point to multi-pointcommunication system;

FIG. 1B shows data packets arriving in order at the receiver of FIG. 1A;

FIG. 2 shows schematically a typical, known, passive optical network(PON) communication system;

FIG. 3 shows a typical receiver unit used in a system as that of FIG. 1;

FIG. 4 shows a typical situation when a Ranging window is allocated fora new user to join the network;

FIG. 5 shows a commonly known packet structure;

FIG. 6 shows a typical existing Ranging method;

FIG. 7 shows a flow chart of a preferred embodiment of the Rangingmethod of the present invention;

FIG. 8 shows an exemplary propagation delay determination processfollowing the method of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a robust, periodic Ranging method, whichrequires no additional hardware for its implementation (except for theability to receive data), and which can always guarantee successfulRanging. A preferred embodiment of the Ranging method of the presentinvention is described with reference to the flow chart shown. Each newuser who wants to join the network transmits a Ranging packet (in theranging window), which consists of a Header and a Payload (described inFIG. 5). After the start of a ranging window in step 702, the MACoptionally activates the AFE (i.e. sends the required control signalsReset APE 310 and Reset CDR 316 to) in step 704, and optionally searchesfor a Preamble (step 706) of the Ranging packet. The “optional” termmeans that APE activation may not be needed for some types of AFEs, orthat the search for a Preamble is skipped. If no Preamble is found in acheck step 708 or if a first timeout period TO1 is exceeded, the APEchecks if a “global” timeout TO3 is exceeded in step 709, and if NO, theprocess returns to step 704. If YES in step 709, the Ranging process isstopped (being unsuccessful). If a Preamble sequence is found in step708, the APE starts looking for a Delimiter of the Ranging packet instep 710. Ranging packet data (such as the abovementioned Preamble andDelimiter) is also referred to herein as “expected data”. If noDelimiter is found in a check step 712 or if a second timeout period TO2is exceeded, the APE checks if TO3 is exceeded in step 713, and if NO,the process returns to step 704. If YES in step 713, the Ranging processis also stopped (being unsuccessful). If a Delimiter is detected in step712, the MAC receives the data, registers the user and estimates theuser's propagation delay (to the receiver). In both cases of failure(YES) in steps 709 and 713, the ranging process needs to be restartedfrom step 702 in the next ranging window.

In an exemplary case, the Ranging packet is set as follows:

Guard time: 32 bits.

Preamble: 600 bits.

Delimiter: 20 bits.

TO1: 200 bits

TO2: 250 bits

TO3: Equal to the size of the ranging window.

FIG. 8 shows schematically an exemplary Ranging process. Since thearrival time of the Ranging packet is not known to the receiver, thereceiver starts operating according to the Ranging method disclosedherein. First, it activates its AFE and start searching for a Preamble.In the figure, the Ranging packet of the single new user is shown as nothaving been found (not arrived) in a period up to T01. Therefore, afterTO1, the process is repeated for a second time, in which the Rangingpacket is still not found. Therefore, after another TO1, the process isrepeated for a third time. In this third “round” (still in the sameranging window) a Preamble of the ranging packet is found, so thereceiver looks for a Delimiter. The Delimiter which follows the Preambleis found, so the ranging process ends successfully: the receiver cancorrectly receive the data, the user can register to the network and thepropagation delay can be estimated.

The process described above and clearly illustrated in FIGS. 7 and 8 is“iterative”, in a cyclical or “repetitive” sense. In contrast with priorart processes, which start once per ranging window, the process hererepeats itself in a ranging window if a match between expected data andreceived data is not found. The iterative (repetitive) process in aranging window ends only if the match is found or if predeterminedtimeouts are exceeded.

After the process described in FIG. 8 is done, the propagation delaydetermination can follow (i.e. the receiver or the user can calculatethe difference between the sending time at the user and the arrivaltime). After this point, the Ranging process is done and both thereceiver and the user are now in a Steady State mode or regime ofoperation. In steady state operation (after the propagation delay isknown to the system), the arrival time of each packet is known to thesystem. For this reason the logic unit can supply control signals (i.e.Reset AFE 310 and Reset CDR 316) to both the AFE and to the CDR exactlywhen required (usually at the beginning of data packet). Note that thesecontrol signals can not be supplied with exact timing when in Rangingmode, because of the unknown propagation delay.

In summary, the method described herein enables to range and find thepropagation delay with almost any existing system and AFE. No dedicatedexpensive hardware is required for the Ranging. The main idea isperiodic trial and error. The receiver activates its AFE and CDR andsearches for a special sequence (a Preamble and Delimiter). If it doesnot find such a sequence, the process (AFE activation and search)periodically start from the beginning. This process ends when findingthe delimiter or after TO3 (unsuccessful). The method, by tuning certainparameters, can meet the requirement of almost any known AFE. It alsoproved itself as a very robust method in a noisy environment.

The disclosed ranging method requires cheaper and easier to implementAFE and CDR components. It is also more robust in case of noisychannels, when a high BER (Bit Error Rate) is expected.

All patents mentioned in this specification are incorporated herein intheir entirety by reference into the specification, to the same extentas if each individual patent was specifically and individually indicatedto be incorporated herein by reference. In addition, citation oridentification of any reference in this application shall not beconstrued as an admission that such reference is available as prior artto the present invention.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made. Inparticular, the Ranging process disclosed herein may work without step704 (activate APE) in cases where the AFE does not require activation(as there are different kinds of AFE). It may also work without step706, directly searching for a Delimiter.

1. In a passive optical network in which data is transferred by packets,a ranging method comprising the steps of: by an optical line terminal:a. After start of a ranging window, periodically comparing received datawith expected data to find a match therebetween; b. in case of a matchfailure, restarting the comparison within the same ranging window untila match is found, wherein said restarting always includes activating ananalog front end of a receiver; and c. upon finding a match, moving to asteady state operation regime.
 2. The method of claim 1, wherein thestep of periodically comparing received data with expected data includessearching an incoming data stream for a known packet preamble until thepreamble is found or until a first timeout is exceeded, wherein theexceeding of the first timeout represents a first match failure.
 3. Themethod of claim 1, wherein the step of periodically comparing receiveddata with expected data includes searching an incoming data stream for aknown packet delimiter until the delimiter is found or until a secondtimeout is exceeded, wherein the exceeding of the second timeoutrepresents a second match failure.
 4. The method of claim 2, wherein thestep of periodically comparing ends in a match failure and wherein thestep of restarting includes restarting the comparing until a globaltimeout is exceeded.
 5. The method of claim 3, wherein the step ofperiodically comparing ends in a match failure and wherein the step ofrestarting includes restarting the comparing until a global timeout isexceeded.
 6. The method of claim 4, wherein the global timeout is equalto the ranging window.
 7. The method of claim 1, wherein said activatingis in response to said restarting.
 8. In a passive optical network inwhich data is transferred by packets, a ranging method comprising thesteps of: by an optical line terminal: a. After start of a rangingwindow: i. activating a receiver analog front end, and then ii.searching for a ranging packet section until the ranging packet sectionis found or until a respective timeout is exceeded; b. if the packetsection is not found or if the respective timeout is exceeded, checkingif a global timeout TO3 is exceeded and if not; c. repeating the stepsof searching and checking within the same ranging window until globaltimeout TO3 is exceeded, wherein said repeating includes alwaysrepeating the activating of the receiver analog front end in addition torepeating the steps of searching and checking.
 9. The method of claim 8,further comprising the step of: d. if the packet section is found in thestep of searching, moving to a steady state operation regime.
 10. Themethod of claim 9, wherein the step of searching for a ranging packetsection includes ranging for a delimiter section.
 11. The method ofclaim 9, wherein the step of searching for a ranging packet sectionincludes searching for two packet sections each associated with arespective timeout.
 12. The method of claim 11, wherein the two packetsections include a preamble section and a delimiter section.
 13. Themethod of claim 9, wherein the global timeout is equal to the rangingwindow.
 14. The method of claim 8, wherein said repeating of saidactivating of said receiver analog front end is in response to saidrepeating of said searching.
 15. A ranging method applied in a passiveoptical network and comprising the steps of: by an optical lineterminal: a. starting a ranging window; and b. within the same rangingwindow, performing an iterative search and comparison process onincoming packet data, the iterative process involving a ranging packet,until at least one ranging packet section is matched with at least onereceived packet data section or until a global timeout TO3 is exceeded,wherein each iteration of said search and comparison process includesactivating a receiver analog front end.
 16. The method of claim 15,wherein the step of performing an iterative search and comparisonprocess includes using a respective timeout at an intermediate stage ofthe ranging window if a match is not found between the at least oneranging packet section and at least one received packet data section inthat intermediate stage.
 17. The method of claim 15, wherein, in eachsaid iteration, said activating of said receiver analog front end is inresponse to commencement of said each iteration.